The present invention is directed generally to acoustic imaging and, more particularly, to an acoustically generated image formed by selected signal components.
The present invention provides a process and an apparatus for enhancing the imaging of subtle structures, such as tumor tissue within a soft tissue matrix. Specifically, the process and apparatus provides a transmissive ultrasonic holography imaging system having an acoustical opaque small element variably placed so as to block the contribution to the image by sound energy transmitted through the object but not scattered by the object being imaged. The present invention further provides a process and apparatus for a transmissive ultrasonic holography imaging system comprising an acoustical opaque planar element having an opening so as to pass unscattered ultrasonic energy (i.e., sound) but to block the contribution to the image by ultrasonic energy that is transmitted through the object and scattered by the object. The present invention further provides an alternate process and apparatus which provides for an acoustical planar element variably placed so as to block all or substantially all of the ultrasonic energy transmitted through the object except that scattered from a selected volume within the object being imaged and at selected forward scattering angles. It is recognized that the nature of the scattering angle relates to the nature of the object being imaged. Thus, in this invention the imaging with selected components refers to imaging with only a selected portion of the ultrasound transmitted through or forward scattered (diffracted) from a structure within an object. The process and apparatus provides for being able to image with only ultrasound scattered at large scattering angles, medium forward scattering angles or low or zero forward scattering angle. Since different characteristics of an object (e.g. lesions in the human breast) forward scatters ultrasonic energy at various angles, by being able to image with ultrasound scattered only selective angles greater and more detailed information can be determined of subtle structures within the object.
The process and apparatus further provides that these two separate image contributions are used and analyzed separately or combined for improved diagnosis of subtle structures. One of the results of utilizing the inventive process provides for improved imaging visualization of subtle objects by providing a means of imaging only with sound scatter from subtle objects because only ultrasound that interferes with the object is transmitted to a holographic detector and reconstructed within the detector. More specifically, the invention provides a process to separately using only specific portions of the transmitted sound wave to make separate images of the object and utilize a combination of such images to provide greater detailed information about subtle structures within the object.
Holography involves combining or interfering an object wave or energy with a reference wave or energy to form an interference pattern referred to as the hologram. A fundamental requirement for the forming of the hologram and the practice of holography is that the initial source of the object wave and reference wave or energy are coherent with respect to the other wave. That is to say, that all parts of both the object wave and the reference wave are of the same frequency and of a defined orientation (a fixed spatial position and angle between the direction of propagation of the two sources). When performing holography the object wave is modified by interference with structure within the object of interest. As this object wave interacts with all points of the object in the path of the wave, the three-dimensional features of the object impart identifying phase and amplitude changes on the object wave. Since the reference wave is an unperturbed (pure) coherent wave, its interference with the object wave results in an interference pattern which identifies the 3-D positioning and characteristics (ultrasonic absorption, diffraction, reflection, and refraction) of the scattering points of the object.
A second process, (the reconstruction of the hologram) is then performed when a coherent viewing source (usually light from a laser) is transmitted through or reflected from the hologram. The hologram pattern diffracts light from this coherent viewing or reconstructing source in a manner to faithfully represent the 3-D nature of the object, as seen by the ultrasonic object wave.
To reiterate, to perform holography, coherent wave sources are required. This requirement currently limits practical applications of the practice of holography to the light domain (e.g., a laser light) or the domain of acoustics (sometimes referred to as ultrasound due to the practical application at ultrasonic frequencies) as these two sources are currently the only available coherent energy sources. Thus, further references to holography or imaging system will refer to the through-transmission holographic imaging process that uses acoustical energies usually in the ultrasonic frequency range and more specifically from 1 to 10 MHz.
In the practice of ultrasound holography, one key process is the generation of the ultrasound, such as a large area coherent ultrasound transducer. A second key process is the projection of the object wave information from a specific volume within the object into the hologram detection plane by means of the ultrasonic lens projection system. A third key process is the detection and reconstruction of the ultrasonic hologram into visual or useful format.
Although other configurations can be utilized, a common requirement of the source transducers for both the object and reference waves is to produce a large area plane wave having constant amplitude across the wave front and having a constant frequency for a sufficient number of cycles to establish coherence. Such transducers will produce this desired wave if the amplitude of the ultrasound output decreases in a Gaussian distribution profile as the edge of the large area transducer is approached. This decreasing of amplitude as the edge is approached, reduces or eliminates the xe2x80x9cedge effectxe2x80x9d from the transducer edge, which would otherwise cause varying amplitude across the wave front as a function distance from the transducer.
In the process of through-transmission ultrasonic holographic imaging, the pulse from the object transducer progresses through the object, then through a focusing lens system and at the appropriate time, the pulse of ultrasound is generated from the reference transducer such that the object wave and reference wave arrive at the detector at the same time to create a interference pattern (i.e., the hologram). For broad applications, the transducers need to be able to operate at a spectrum or bandwidth of discrete frequencies. Multiple frequencies allow comparisons and integration of holograms taken at selected frequencies to provide an improved image of the subtle changes within the object.
A hologram can also be formed by directing the object wave through the object at different angles to the central axis of the lens system. This is provided by either positioning or rotating the object transducer around the central axis of the lens system by using multiple transducers positioned such that the path of transmission of the sound is at an angle with respect to the central axis of the lens system.
With a through-transmission imaging system, it is important to determine the amount of resolution in the xe2x80x9czxe2x80x9d dimension that is desirable and achievable. Since the holographic process operates without limits of mechanical or electronic devices to detect and form the image, but rather reconstructs images from wave interactions, the resolution achievable can approach the theoretical limit of xc2xd the wavelength of the ultrasound used. However, the amount of information displayed for the user in this situation may be too great. It may be desirable to limit the xe2x80x9czxe2x80x9d direction image volume so that one can xe2x80x9cfocusxe2x80x9d in on one thin volume slice and thereby reduce the amount of data. Thus, it is of value to develop a means for projecting a planar slice within a volume into the detector plane. One such means is a large aperture ultrasonic lens system that will allow the imaging system to xe2x80x9cfocusxe2x80x9d on a plane within the object. Additionally, this lens system and the corresponding motorized, computer controlled lens drive will allow one to adjust the focal plane and at any given focal plane to be able to magnify or demagnify at a selected z dimension position (i.e., a zoom lens).
The image is detected and reconstructed at the detector. Standard photographic film may be used for the recording of light holograms and the 3-D image reconstructed by passing laser light through the film or reflecting it from the hologram pattern embossed on the surface of an optical reflective surface. However, there is no equivalent xe2x80x9cfilmxe2x80x9d material to record the intricate phase and amplitude pattern of a complex ultrasonic wave. One of the most common detectors uses a liquid-air surface or interface to record, in a dynamic way, the ultrasonic hologram formed. The sound energy at the frequency of ultrasound (above range of human hearing) will propagate with little attenuation through a liquid (such as water) but cannot propagate through air. At these higher frequencies (e.g., above 1 MHz) the ultrasound will not propagate through air because the wavelength of the sound energy is so short [xcex(wavelength)=v(velocity)/ƒ(frequency)]. The density of air (approximately 0.00116 g/cm3) is not sufficient to couple these short wavelengths and allow them to propagate. On the other hand the density of a liquid (e.g., water) is a favorable media to couple and propagate such wavelengths. For example, the velocity of sound in air is approximately 346 meters/second whereas in water it is approximately 1497 meter/second. Thus, for water, both the density (1 g/cm3) and the wavelength (xcx9c1.5 mm at 1 MHz) are significantly large that ultrasound can propagate with little attenuation. In contrast, for air both the density (0.00116 g/cm3) and wavelength (0.346 mm at 1 MHz) are sufficiently small such that the energy at these ultrasonic frequencies will not propagate.
Thus, when ultrasound propagating in a liquid encounters a liquid-air interface the entire amount of the energy is reflected back into the liquid. Since ultrasound (or sound) propagates as a mechanical force it is apparent that the reflection (or changing direction of propagation) will impart a forward force on this liquid-air interface. This force, in turn, will distort the surface of the liquid. The amount of surface distortion will depend upon the amplitude of the ultrasound wave at each point being reflected and the surface tension of the liquid. Thus, the pattern of the deformation is the pattern of the phase and amplitude of the ultrasonic wave.
In this manner, the liquid-air interface can be readily used to provide a near real-time recorder (xe2x80x9cfilm equivalentxe2x80x9d) for an ultrasonic hologram. The shape of the surface deformation on this liquid-air detector is the representation of the phase and amplitude of the ultrasonic hologram formed by the interference of the object and reference ultrasonic waves.
The greatest value of the ultrasonic holographic process is achieved by reconstructing the hologram in a usable manner, usually in light, to make visible the structural nature of the initial object. In the case of a liquid-air interface, the reconstruction to achieve the visible image is accomplished by reflecting a coherent light from this liquid-air surface. This is the equivalent process to reflecting laser light from optically generated hologram that is embossed on the surface of a reflecting material (e.g., thin aluminum film).
The reflected light is diffracted (scattered) by the hologram to diffracted orders, each of which contains image information about the object. These diffracted orders are referred to as xc2x1nth orders. That part of the reconstructing light that does not react with the hologram is referred to as zero order and is usually blocked so that the weaker diffracted orders can be imaged. The higher the diffracted order the greater is the separation angle between the zero order of reflected light. Once reconstructed, the image may be viewed directly, by means of a video camera or through post processing processes.
Ultrasonic holography as typically practiced is illustrated in FIG. 1. A plane wave of sound 1a (i.e., ultrasound) is generated by a large area object transducer 1. Such a transducer is described in U.S. Pat. No. 5,329,202. The sound is scattered (i.e., diffracted) by structural points within the object. The scattered sound 2a from the internal object points that lie in the focal plane 2 are focused (i.e., projected) into a hologram detector plane 6 of a hologram detector 7. The focusing is accomplished by an ultrasonic lens system 3, which focuses the scattered sound into the hologram detector plane 6 and the unscattered sound into a focal point 4. U.S. Pat. No. 5,235,553 describes an ultrasonic lens that may be satisfactorily used for the ultrasonic lenses illustrated as the lens system 3 in FIG. 1. The ultrasonic lens system 3 also allows the imaging process to magnify the image (i.e., zoom) or change focus position. U.S. Pat. No. 5,212,571 illustrates a lens system that can magnify the image and change focus position and may be used satisfactorily for the lens system 3.
Since the focal point 4 of the unscattered sound is prior to the hologram detector plane 6, this portion of the total sound again expands to form the transparent image contribution (that portion of the sound that transmitted through the object as if it were transparent or semi-transparent). In such an application, an ultrasound reflector 5 is generally used to direct the object sound at a different angle, thus impinging on the hologram detector plane 6, which usually contains a liquid that is deformed by the ultrasound reflecting from the liquid-air interface. In an exemplary embodiment, the base of the hologram detector 7 is made to be parallel with the ground so that the thickness of the fluid below hologram plane 6 remains at a constant value.
When a reference wave 8 and the object wave are simultaneously reflected from the hologram detector 7, the deformation of the liquid-air interface is the exact pattern of the ultrasonic hologram formed by the object wave (la combined with 2a) and the xe2x80x9coff-axisxe2x80x9d reference wave 8.
This ultrasonic hologram formed on the detector plane 6 is subsequently reconstructed for viewing by using a coherent light source 9, which may be passed through an optical lens 10, and reflected from the holographic detector plane 6. U.S. patent application Ser. No. 09/589,863 describes a hologram detector suitable for use as the hologram detector 7 illustrated in FIG. 1.
This reflected coherent light contains two components. The first component is light that is reflected from the ultrasound hologram that was not diffracted by the ultrasonic holographic pattern, which is focused at position 11 and referred to as undiffracted or zero order light. The second component is light that does get diffracted from/by the ultrasonic hologram is reflected at an xe2x80x9coff-axisxe2x80x9d angle from the zero order at position 12 and referred to as the xe2x80x9cfirst orderxe2x80x9d image view when passed through a spatial filter 13. It is noted that this reconstruction method produces multiple diffraction orders each containing the ultrasonic object information. Note also both + and xe2x88x92 multiple orders of the diffracted image are present and can be used individually or in combinations to view the optical reconstructed image from the ultrasonically formed hologram by modifying the spatial filter 13 accordingly.
That portion of the ultrasound wave that passes through the imaged object without interference with the object can be a major contributor in xe2x80x9csemitransparent objectsxe2x80x9d (that is, an object that scatters a small portion of the sound waves passed through the object). Since many objects of interest can be rather transparent to sound, (e.g. human soft tissue normal structures and tumor tissue of solid tumors) a major portion of the sound source passes through the object and forms a background hologram that diffracts light to form a bright and strong white light contribution. When one wants to detect and determine the characteristic of subtle changes in the object (e.g., determining tissue characteristics) this background bright image contribution can overpower the resolution of small and subtle contributions of tissue change. Therefore, there is a need in the art to improve resolution characteristics of transmissive ultrasonic imaging so as to be able to distinguish subtle differences within the object (i.e., so as to be able to image tumor tissue within surrounding soft breast tissue).
Furthermore, there is a need in the art to improve image quality by recognizing and utilizing the effects of diffraction generated by internal structures within the object. This need is particularly strong for breast cancer screening techniques that now utilize invasive mammography (providing the patient with exposure to radiation from X-Ray imaging) and yet do not have sufficient sensitivity to certain types of cancerous conditions e.g. cancer not exhibiting calcification or in radiographic dense breasts of young women. The present invention provides this, and other advantages, as will be apparent from the following detailed description and accompanying figures.
The present invention relates to acoustically generated images of an imaged object. A typical acoustically generated image is generated using one or more of the components resulting from a through transmitted ultrasonic wave forward scattered from the structure within the object being imaged.
The acoustic signal generated by transducers passes through and interacts with an object to produce an acoustic signal having a diffracted component and a nondiffracted component. The acoustic image of the present invention is formed by selected portions of the sound either scattered from or passed through the object being imaged. In use of the present invention, images are generated with either the scattered ultrasound component only, the ultrasound component that is not scattered, a combination thereof or ultrasound forward scattered from structures within the object at a selected angle which may be referred to as at a selected spatial frequency. In one embodiment, the acoustic signals may be ultrasonic acoustic signals. In one embodiment, the acoustically generated image may be a holographic image. The holographic image may be viewed with light (such as a laser light) interaction with the ultrasonic hologram. The holographic image is generated through the interaction of light and an acoustic interference pattern. The acoustic interference pattern may be formed at an liquid-air interface upon which the light is directed.
In an exemplary embodiment, the acoustically generated image information may be constructed by ultrasound that is scattered by internal structures of the object and at a selected angle i.e. at a selected spatial frequency.